If you look at what MS are and do you will see that mobile hardware is not there thing at all,
if i was CEO of MS and i wanted to go big in mobile software then i would look to buy a company that manufacturs mobile hardware. They like everyone else in the world have been watching Nokia and with Nokia obviously failing mainly because of there software weakness, MS obviously seen what was happening with Nokia knowing full well the potential of a merger or buyout. Buying out Nokia is not an option unless you have THAT much capitol to use so they waited for the right time to jump in with the merger offer bringing the 2 together as one basically.
Nokia WILL benefit no mater what in the long term as MS have a huge past record behind them and can sustain this for a very long time.
MS on the other hand needed to develop further in another field so with mobile hardware technology improving tremendously, they have made the move probably at the right time on mobile software now, having the right company to develop the hardware.
This merger will not be short lived in any way or form and people who think otherwise do not look deep enough into the 2 companies. Something good has to happen on the law of averages alone so my mind is set on a wait and see basis but i bet we will not have to wait too long.